REDUCTION OF THE NUMBER OF PCI PROTECTED FUEL RODS IN A REACTOR CORE

Abstract: The invention relates to a fuel assembly for a boiling water reactor in which the number of fuel rod positions is greater than 80, and to a fuel assembly for a boiling water reactor in which the number of fuel rod positions is smaller than 65, and to a fuel assembly for a pressurized-water reactor. A fuel assembly comprises a plurality of fuel rods each of which comprises a nuclear fuel surrounded by a cladding tube. The fuel assembly is located adjacent to one or more control rod positions (10). To protect a fuel rod against PCI damage, the inside of the cladding tube is provided with a protective layer of metal. Only fuel rods (7a) in especially exposed positions near a control rod position are provided with cladding protection. The other fuel rods (7b) have no cladding protection. The invention also relates to a core for a boiling water reactor and a pressurized-water reactor in which all the fuel assemblies, which either contain or are arranged adjacent to a control rod which during normal operation at some time is inserted into the core, comprise fuel rods with said protective layer. The total number of fuel assemblies in the entire core with said protective layer is smaller than 1.2 x C x M, where C is the average number of operating cycles during which a fuel assembly is allowed to sit in the core, and M is the number of fuel assemblies which either contain or are arranged adjacent to a control rod which during normal operation at some time is inserted into the core.

Description: Reduction of the number of PCI protected fuel rods in a reactor core TECHNICAL FIELD The present invention relates to a fuel assembly comprising a plurality of fuel rods in which each fuel rods comprises a cladding tube which encloses a nuclear fuel. The fuel assembly is arranged adjacent to one or more control rod positions and contains a first group of fuel rods provided with PCI protection and a second group of fuel rods without PCI protection. The first group of fuel rods lies nearer the central part of a control rod position than does the second group. The invention also relates to a core comprising a plurality of fuel assemblies and a plurality of control rods. The con¬ trol rods can both be inserted into and withdrawn from the core. In a first group of control rods at least certain con- trol rods are partially inserted into the core during normal operation of the reactor. In a second group of control rods all the control rods are withdrawn from the core during normal operation. BACKGROUND ART, PROBLEMS A core in a nuclear reactor comprises a plurality of fuel assemblies. These are arranged vertically in the core in a certain spaced relationship to each other. A fuel assembly comprises a plurality of vertical fuel rods which contains a stack of circular-cylindrical pellets of a nuclear fuel arranged in a cladding tube. The core is immersed into water which serves both as a coolant and as a neutron moderator. The reactor core also comprises a number or control rods which control the reactivity of the core, and hence the output power thereof, by being inserted into and withdrawn from the core. The location of the control rods when they are fully inserted into the core, that is, the positions of the control rods in a cross section of the core - herein¬ after referred to as control rod positions - are different for different reactors. By a control rod position is meant the entire space occupied by a control rod fully inserted into the core. Some of the control rods are withdrawn from the core during operation. The other control rods are inser¬ ted to different levels in the core. These levels vary during the operating cycle of the reactor. During use of the nuclear fuel, a problem arises concerning damage to the cladding, caused by a combination of chemical and mechanical interaction between the nuclear fuel and the cladding, so-called PCI damage (PCI = pellet-clad interaction) . As cladding tubes, normally thin-walled tubes of zirconium-based alloys are used, which are known under the trade name zircaloy. These alloys irradiation-harden under neutron irradiation. The irradiation hardening leads to an embrittlement of the material. When the power rapidly increases, mechanical stresses arise due to differences in extension between the fuel and the cladding. These stresses may lead to cracks in the embrittled cladding tube. The mechanical stresses are reinforced by high friction between the fuel and the cladding. The higher the temperature at the centre of a rod, the greater the difference in extension between fuel and cladding and the greater the risk of damage caused by mechanical interaction. At high temperatures at the centre of the rod, fission products, such as iodine, are released. These fission products subject the inside of the cladding tube to a chemical influence, so-called stress corrosion, which reduces the mechanical strength of the material . To counteract cladding damage caused by chemical and mechanical interaction, it is known to provide the cladding tube internally with a protective layer of metal, a so- called liner, which has good resistance to hardening when subjected to neutron irradiation and which in itself is a relatively soft material. Because of the properties mentioned, the liner can be deformed plastically and thereby protect the cladding tube against stresses, caused by the nuclear fuel, which arise during operation in case of rapid changes of power in combination with high temperatures at the centre of the rod. The liner may, for example, be a pure zirconium or of some zirconium alloy. Experience has shown that, for example in connection with repairs and service of a nuclear reactor, debris, for example metal chips, may enter and then move with the water which circulates through the core. The most frequent type of fuel damage in nuclear reactors today is abrasion damage to the cladding tube caused by debris in the cooling water. One problem with liners is that they generally have a low resistance to corrosion by water and water steam. In addition, in case of corrosion of the liner, corrosion products are formed which accelerate the embrittlement of the cladding tube. When external damage to the cladding tube arises, for example abrasion damage, water and water steam may penetrate into the interior of the cladding tube and the liner starts corroding. In case of external damage to the cladding tube, a fuel rod with a liner exhibits considerably more serious damage than a fuel rod without a liner. An additional disadvantage of using cladding tubes with liners is that the mechanical strength is inferior compared with a cladding tube without a liner. In recent years the fuel manufacturers have increasingly changed to fuel assemblies with a greater rod density, that is, the number of fuel rod positions per unit of surface is greater than before. This also means that the rods have decreased in diameter. Since the temperature at the rod centre depends on the diameter of the rod, this means that the temperature at the rod centre has been reduced compared with previously. It has been noted that the extent of PCI damage has decreased since the introduction of fuel assemblies with a greater rod density. This decrease has led to a questioning whether the advantages of a liner really outweighs its disadvantages. It has also proved that the PCI damage primarily arises in those rods which are located nearest the control rod positions. In Japanese application JP, A, 1304389 the following problem is described. To prevent PCI damage, a compound of, for example, a metal oxide or a clay material has been added to the fuel pellets. From the PCI point of view, this is stated to function well, but from an economical point of view it is a great disadvantage since these additives generally have a relatively great neutron absorption cross section, that is, their ability to absorb neutrons is great. In addition, these additives dilute the fuel, which also contributes to an inferior fuel economy. This problem has been solved by placing rods with addition of PCI protecting substances in the outermost rows only, immediately adjoining a control rod position. A liner is most often made of pure zirconium or some zirconium alloy and has a very low neutron absorption cross section. Therefore, no problem corresponding to that described in the Japanese application exists with PCI protection in the form of a liner. In the Japanese application, no fuel configurations other than an 8x8 fuel, that is, with 64 fuel rod positions, are mentioned. The embodiment described has PCI protection, in the form of additive to the fuel, only in the outer rows which immediately adjoin a control rod position. One problem with this embodiment is that it does not provide sufficient protection against PCI damage if applied to an 8x8 fuel with PCI protection in the form of a liner. SUMMARY OF THE INVENTION , ADVANTAGES The object of the invention is to reduce the risk of serious abrasion damage in a reactor core without increasing the risk of PCI damage. A reduced risk of serious abrasion damage is achieved by reducing the number of fuel rods with liners in the core. According to the invention, fuel rods with liners are arranged only in certain positions which are particularly exposed to PCI damage, whereas fuel rods without liners are arranged in the other positions. The invention aims to provide a fuel assembly for a boiling water reactor in which the number of fuel rod positions in the fuel assembly is greater than 80, and a fuel assembly for a boiling water reactor in which the number of fuel rod positions in the fuel assembly is smaller than 65. The invention also aims to provide a fuel assembly for a pressurized-water reactor. The invention also aims to provide a core in a boiling water reactor and a core in a pressurized-water reactor, in which the number of fuel rods with liners is reduced without the risk of PCT damage increasing. What characterizes the different fuel assemblies and the two cores according to the invention will become clear from the appended claims. When the number of fuel rods with liners is reduced, this also reduces the risk of serious cladding damage caused by corroding liners in connection with external abrasion damage. The invention is therefore particularly advantageous for fuel assemblies with a high rod density (more than 80 fuel rod positions) , since these involve - a reduced risk of PCI damage, - an increased risk of abrasion damage, and - a higher manufacturing cost because of a larger number of rods as compared with fuel assemblies with a lower rod density. To obtain sufficiently good PCI protection for assemblies with a lower rod density (less than 65 fuel rod positions), more fuel rods with liners are needed in relation to the total number of fuel rods in the assembly compared with the situation for assemblies with a higher rod density. This means that the invention is not equally advantageous for a fuel with less than 65 fuel rod positions. BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 shows a horizontal section of part of a reactor core for a boiling reactor with vertical fuel assemblies according to the prior art . Figure 2 shows a fuel rod in a vertical section. Figures 3a - 3g shows in horizontal sections a number of embodiments of a fuel assembly according to the invention. Figures 4a - 4f show in horizontal sections alternative embodiments of a fuel assembly according to the invention. Figures 5a - 5c show horizontal sections of a number of embodiments of a fuel assembly for a pressurized-water reactor according to the invention. Figure 6 shows a horizontal section through a core in a boiling water reactor according to the invention. Figure 7 shows a horizontal section through a core in a pressurized-water reactor according to the invention. DESCRIPTION OF THE PREFERRED EMBODIMENTS Figure 1 shows horizontal section of part of a reactor core for a boiling reactor with fuel assemblies according to the prior art. The fuel assemblies have a substantially square cross section and are arranged vertically in the core in a certain spaced relationship to each other. This leads to the formation of a check pattern of vertically standing gaps between the fuel assemblies. The section includes nine fuel assemblies la-li. The total number of fuel assemblies in a whole cross section amounts to several hundred. The fuel assembly is enclosed in a fuel channel 2 which is provided with a hollow support member 3 of cruciform cross section. The support member has four hollow wings 4 which form a central channel 5 where moderator water circulates. The fuel channel with support members is divided into four vertical channel-formed parts with at least substantially square cross section, each one comprising a sub-bundle 6. Each sub-bundle 6 contains a number of fuel rods 7. The rods are arranged in a lattice in which each rod 7 is included in two rows perpendicular to each other. The rods are retained in their positions by means of so-called spacers (not shown) , placed between top tie plates and bottom tie plates (also not shown) on the fuel assembly. The spaces 8 between the fuel rods within each sub-assembly 6 are traversed by water. The gaps 9a and 9b between the fuel assemblies are also traversed by water. The reactor core also comprises a number of control rods, lla-lld, placed in parallel with the fuel assemblies in the vertically extending gaps. The control rods have a cruciform cross section with four blades, extending perpendicularly to each other from a central part 12, which form a right-angled cross. In the embodiment shown, the gaps 9b into which the control rods can be inserted are wider than the gaps 9a where there are no control rods. By a control rod position 10 for a control rod for a boiling water reactor is meant the entire cruciform space which is occupied by a control rod fully inserted into the core. The fuel assemblies are arranged in a symmetrical lattice with each fuel assembly included in two rows of fuel assem ¬ blies perpendicular to each other. The control rods are arranged with each one of their blades between two fuel assemblies located in the same row, such that each control rod together with four fuel assemblies, arranged around their blades, form a unit, hereinafter referred to as a supercell, with at least substantially square cross section. In Figure 1, the control rod lie forms together with the fuel assemblies Id, le, lg and lh a supercell. As is clear from Figure 2, each fuel rod 7 accommodates a cladding tube 20 and a large number of circular-cylindrical pellets 21 stacked one above the other in the axial direction of the tube. The lowermost pellet rests rigidly on a bottom plug 22 welded to the lower end of the rod, and the uppermost pellet is pressed downwards by a spiral spring 23 which is tightened against a top plug 24 welded to the upper end of the tube. Figure 3a shows one embodiment of the invention. In contrast to the fuel assembly shown in Figure 1, the fuel assembly 1 is provided with an enlarged centre in that the cruciform channel is formed with internal corner reductions (prior art according to Swedish patent 454 822) . The fuel assembly comprises a first group of fuel rods provided with PCI pro¬ tection 7a to prevent cladding damage caused by the inter¬ action of the nuclear fuel with the cladding tube, as well as a second group of fuel rods without such protection. The first group includes all the fuel rods which adjoin the con¬ trol rod position 10 as well as the three rods which have the shortest distance to the cruciform centre without imme¬ diately adjoining the control rod position. The PCI protection consists of a protective layer, a so- called liner, of a zirconium alloy arranged on the internal surface of the cladding tube. Such a liner is described in Swedish patent application 9200584-2. The fuel rods are placed in a substantially regular lattice. By a fuel rod position is meant a position in the lattice. All the fuel rod positions in the lattice need not be occu- pied by fuel rods. In certain fuel assemblies, a number of fuel rods are replaced by one or more water channels . The introduction of a water channel changes the number of fuel rods but not the number of fuel rod positions. Fuel assem- blies with the same external dimensions may have different numbers of fuel rod positions. The rod density of a fuel assembly is defined as the number of fuel rod positions per unit of surface. In an assembly with a high rod density, the fuel rods are generally thinner than the fuel rods in an assembly with a smaller rod density. The number of fuel rod positions per assembly may, for a boiling reactor, vary from 7x7, that is, 49, in older types of assemblies to 11x11, that is, 121, in certain modern assemblies. The invention is particularly suitable for assemblies in which the number of fuel rod positions is greater than or equal to 9x9, that is, 81. Since the risk of cladding damage is greatest for fuel rods located near the control rod blades and above all in the vicinity of the control rod centre, it is those fuel rods which are located in these exposed positions which are to be PCI protected. How great the need of protection is, that is, how many fuel rods that need liners, is dependent on several different factors which may vary between different types of fuel and different reactor plants. Examples of such factors are the width of the control rod gaps 9b, the ability of the control rods to absorb neutrons, and the way in which the control rods are inserted into the core. Control rods which are moved in and out in a slow continuous movement do not cause an equally fast power change as a control rod which is inserted in stages, in which the movement between each stage takes place rapidly. When using fuel with a high enrichment the risk of PCI damage increases and the need of protecting the fuel rods increases. In a fuel with a high rod density, the need of protecting the rods decreases. Figures 3a-3g show a number of different embodiments of the invention in which the number of fuel rods in the first and second groups vary. Which rods are included in the two groups are clear from the figures. If the risk of cladding damage is judged to be high, an alternative with many rods with liners, 7a, is chosen, for example Figure 3c or Figure 3d. If the risk of cladding damage is judged to be low, an alternative with few rods with liners is chosen, for example Figure 3f or Figure 3g. Common to all the embodiments is that the first group of fuel rods lies nearer to the central part of the control rod position than does the second group. Individual fuel rods in the second group may lie nearer to the central part of the control rod position than certain fuel rods in the first group. By a group is meant the group as an entirety. The mean distance of a group to the central part of a control rod position is calculated as the sum of the distance between each fuel rod associated with the group and the central part of the nearest control rod position, divided by the number of fuel rods in the group. When a first group lies nearer the central part of a control rod position than a second group, the intention is that the mean distance of the first group to the central part of the control rod position is smaller than the mean distance of the second group. Figures 4a-4f show other fuel assemblies which are suitable for use according to the invention. Each type of fuel assem¬ bly is provided in the figure with a first group of fuel rods provided with liners 7a, and with a second group of fuel rods without liners. The number of fuel rods with liners in each assembly is only an example, and, in prin¬ ciple, all the embodiments described in Figures 3a-3g are . also applicable to these fuel assemblies. Assemblies with a high rod density normally are provided with a smaller number of rods with liners, relatively seen, than assemblies with a lower rod density. The embodiments in Figures 4c, 4d and 4f are not suitable for an 8x8 fuel . Figure 4a shows a fuel assembly without an internal water channel. The number of fuel rod positions is 64. Figure 4b shows a fuel assembly provided with a central cruciform channel 5 through which water is passed in a vertical direction from below and upwards through the assembly. The number of fuel rod positions is 64. Figure 4c shows a fuel assembly provided with a central cruciform channel 5. The number of fuel rod positions is 100. Figure 4d shows a fuel assembly provided with an internally arranged vertical channel 30 through which water is passed in a vertical direction from below and upwards through the assembly. The channel has a substantially square cross section. The number of fuel rod positions is 81. Figure 4e shows a fuel assembly provided with an internally arranged vertical channel 31 through which water is passed from below and upwards through the assembly. The number of fuel rod positions is 64. Figure 4f shows a fuel assembly which is provided with two centrally arranged vertical water rods 32 through which water is passed from below and upwards through the assembly. The water rods have a diameter which is somewhat larger than the diameter of the fuel rods and is formed with a substan¬ tially circular cross section. The number of fuel rod posi¬ tions is 81. The invention may be applied also to fuel assemblies for a pressurized-water reactor (PWR) . Figure 5a shows a fuel assembly for a pressurized-water reactor according to the invention, which comprises control rod positions 40, an instrumentation tube 41, fuel rods with liners, 42a, and fuel rods without liners, 42b. The control rod positions for a pressurized-water reactor are located inside the fuel assemblies, in contrast to a boiling reactor which has the control rod positions between the assemblies. The control rods which are 24 in number have a substantially circular cross section and a diameter which is somewhat larger than the diameter of the fuel rods. The fuel rods which imme¬ diately adjoin a control rod position form a first group of fuel rods which are provided with liners, 42a. For each control rod position there are four immediately adjoining fuel rods. The other fuel rods have no liners, 42b, and are included in a second group of fuel rods. Figure 5b shows another embodiment of the invention in which all fuel rods adjoining a control rod position form a first group of fuel rods which are provided with liners, 42a. For each control rod position there are eight immediately adjoining fuel rods. The other fuel rods have no liners, 42b, and are included in a second group of fuel rods. In this embodiment, the number of fuel rods in the first group is greater than the number of fuel rods in the second group. Figure 5c shows another type of fuel assembly for a pressurized-water reactor. The control rod positions 50 which are five in number have a substantially circular cross section and a diameter which is considerably larger than the diameter of the fuel rods. All the fuel rods adjoining a control rod position form a first group of fuel rods which are provided with liners 42a. For each control rod position there are twelve immediately adjoining fuel rods. The other fuel rods have no liners 42b and are included in a second group of fuel rods. To be able to distinguish between fuel rods with and fuel rod without liners, the top plug or the bottom plug is suitably provided with some form of marking. In a boiling water reactor there is a small number of specially selected control rods, so-called regulating rods, which are used during operation to compensate for the burnup of the fuel. The other control rods are then fully withdrawn from the core. At the beginning of an operating cycle, that is, the time between two refuellings, the regulating rods are at least partially inserted into the core. To compensate for the burnup of the fuel, the regulating rods are with¬ drawn successively during the operating cycle. The control rods which do not belong to the regulating rods are used to start and stop the reactor. A supercell which contains a regulating rod is referred to hereinafter as a control cell. The duration of an operating cycle is usully one year, but it varies and may in certain reactors be up to two years. The number of operating cycles during which the fuel, on average, is located in the core before refuelling also varies between different reactors. If, for example, an operating cycle is one year and the average time for a fuel assembly in the core is 5 cycles, about one-fifth of the fuel assemblies need to be replaced by new ones each year. To reduce the risk of fuel damage to the fuel assemblies in the control cells, they are charged with fuel which has a relatively high burnup. To obtain a suitable burnup, these assemblies have been placed during one or more operating cycles in some of the other super¬ cells. Normally, the fuel assemblies in the control cells usually sit there during one operating cycle only. Figure 6 shows a horizontal section through a core in a boiling water reactor. The core comprises a large number of supercells 60a, 60b, arranged in a symmetrical lattice with each supercell included in two rows of supercells perpendi- cular to each other. Nine of the supercells comprise control elements, so-called control cells, 60b. The other super¬ cells, 60a, comprise control rods which are fully withdrawn during normal operation. Each supercell comprises four fuel assemblies 61a, 61b. The fuel assemblies in the control cells run a higher risk of PCI damage than the other supercells. For a fuel with a high rod density, the risk of PCI damage in the other super- cells is so small that the fuel rods may be completely without liners. The fuel assembly 61a is completely without a liner on the rods. In the control cells, the risk of PCI damage is still so high that a liner is necessary. For a fuel with a high rod density, it is sufficient with a liner on fuel rods which are located in especially exposed posi¬ tions near a control rod position. The fuel assemblies in the control rods, 61b, may, for example, be some of the assemblies in Figures 3a-3g or 4a-4f. It is also possible that all the fuel rods in the fuel assembly 61b have liners. According to the invention, the risk of serious abrasion damage is reduced, while at the same time the risk of PCI damage is kept low, if at least certain fuel rods in those fuel assemblies which are included in the control cells are provided with liners, while at the same time as many as possible of the other fuel assemblies have no liners. Fuel assemblies which include fuel rods with liners are herein¬ after referred to as liner assemblies. The fact that the fuel assemblies usually are only allowed to sit during one operating cycle in the control cells, and that these, in addition, shall have a certain burnup, makes it necessary for the core upon each refuelling to be supplied with, on average, at least as many liner assemblies as the total number of fuel assemblies in all the control cells, that is, 4 x N, where N is the number of control cells in the core. In the core in Figure 6, this means that the core must be supplied with at least 4 x 9 = 36 liner assemblies upon each refuelling. The number of control cells varies usually between five and twelve depending on the operating conditions, but smaller and greater values may occur. The total number of liner assemblies in the core is then at least C x 4 x N, where C is the average number of operating cycles during which a fuel assembly sits in the core. To be certain that there are always liner assemblies enough for all the control cells, it is suitable to have a few extra liner assemblies in the core. The number of liner assemblies must not be too great since this increases the risk of serious abrasion damage. According to the invention, the number of extra liner assemblies in the core may be 20% at most. The total number of liner assemblies in the core may be l.2 x C x 4 x at most, and these are substantially evenly distributed with respect to burnup time. The core in Figure 6 contains 444 fuel assemblies and nine control cells. The average number of operating cycles during which a fuel assembly sits in the core is 5. The number of fuel assemblies which are replaced per operating cycle is about 90, of which 4 x 9 = 36 must be liner assemblies. The total number of liner assemblies in the core must be at least 5 x 36 = 180, but may be at most 1.2 x 180 = 216. With a larger core, for example 700 fuel assemblies, and a smaller number of control cells, for example 5, the number of liner assemblies decreases and hence the risk of serious cladding damage. Figure 7 shows a horizontal section through a core in a pressurized-water reactor comprising a plurality of fuel assemblies, 70a, 70b, 70c. Certain of the fuel assemblies 70, 70c comprise a number of control rods. A first group of fuel assemblies 70b comprises control rods which during operation are used for power control. This first group is normally divided into several sub-groups for, for example, power level control and power shaping. The division of the control rods into sub-groups varies between different reactors. A second group of fuel assemblies 70c comprises control rod which are all, during normal operation, with¬ drawn from the core and only used for starting and stopping the reactor. The other fuel assemblies 70a do not comprise any control rods. According to the invention, all the fuel assemblies 70b in the first group comprise fuel rods in which the inside of the cladding tube is provided with a liner, and the total number of fuel assemblies in the whole core which comprises fuel rods with liners shall be smaller than 1.2 x C x N, where N is the average number of operating cycles during which a fuel assembly sits in the core, and N is the number of fuel assemblies in the first group. In this embodiment, the fuel assemblies in the first group only have liners on those fuel rods which are placed in particularly exposed positions in the vicinity of a control rod position. Suitable fuel assemblies 70b to use in the first group are shown in Figures 5a, 5b and 5c. By normal operation is meant operation with the power variations which are normally required by the reactor to adapt to the requirement of the network. Normal operation comprises daily load cycling, weekend load cycling, and frequency control.

Claims: CLAIMS 1. A fuel assembly for a boiling water reactor comprising a plurality of fuel rods in which each fuel rod comprises a cladding tube (20) which encloses a nuclear fuel (21), wherein the fuel assembly is arranged adjacent to a control rod position (10), and wherein the fuel assembly contains a first group of fuel rods (7a) provided with cladding protection against cladding damage caused by the interaction of the nuclear fuel with the cladding tube, as well as a second group of fuel rods (7b) which have no such cladding protection, and wherein said first group of fuel rods lies nearer the central part of the control rod position than does said second group, characterized in that - the number of fuel rods positions in the fuel assembly is greater than 80, and that - the cladding protection consists of a protective layer of metal on the inside of the cladding tube. 2. A fuel assembly for a boiling water reactor with less than 65 fuel rod positions, comprising a plurality of rows of fuel rods perpendicular to each other, wherein each fuel rod comprises a cladding tube (20) which encloses a nuclear fuel (21) , wherein the fuel assembly is arranged adjacent to a control rod position (10) in such a way that two rows of fuel rods perpendicular to each other adjoin the control rod position, and the fuel assembly contains a first group of fuel rods (7a) provided with cladding protection against cladding damage caused by the interaction of the nuclear fuel with the cladding tube, as well as a second group of fuel rods (7b) which have no such cladding protection, said first group comprising a plurality of fuel rods in the two rows adjoining the control rod position, characterized in that - the cladding protection is a protective layer of metal on the inside of the cladding tube, and that - said first group also comprises at least three fuel rods in rows inside said rows adjoining the control rod position. 3. A fuel assembly for a pressurized-water reactor comprising a plurality of fuel rods in which each fuel rod comprises a cladding tube (20) which encloses a nuclear fuel (21) and wherein the fuel assembly comprises a plurality of control rod positions (40, 50), characterized in that - the fuel assembly contains a first group of fuel rods (42a, 51a) provided with cladding protection against cladding damage caused by the interaction of the nuclear fuel with the cladding tube, as well as a second group of fuel rods (42b, 51b) which have no such cladding protection, and that - said first group of fuel rods lies nearer a control rod position than said second group, and that - the cladding protection consists of a protective level of metal on the inside of the cladding tube. 4. A fuel assembly according to claim 3, characterized in that each fuel rod in the first group lies nearer a control rod position than any rod in the second group. 5. A fuel assembly according to claim 1, 2 or 3 , characterized in that fuel rods with said protective layer are provided with a marking which distinguishes them from fuel rods without said protective layer. 6. A core in a boiling water reactor comprising a plurality of fuel assemblies, each one consisting of a bundle of fuel rods surrounded by a fuel channel, wherein each fuel rod comprises a cladding tube (20) which encloses a nuclear fuel (21) , and the core comprises, in addition, a plurality of control rods (lla-lld), which may both be inserted into and withdrawn from the core, and each of which comprise four vertical blades which form a right-angled cross, the control rods being arranged such that each control rod together with four fuel assemblies arranged around its blades forms one unit, a supercell (60a, 60b) , wherein of a first group of control rods, during operation of the reactor, certain are at least partially inserted into the core, and wherein of a second group of control rods, during normal operation, all are withdrawn from the core, characterized in that - each fuel assembly (61b) which is included in a supercell (60b) which contains a control rod from the first group comprises fuel rods (7a) in which the inside of the cladding tube is provided with a protective layer of metal for preventing cladding damage caused by the interaction of the nuclear fuel with the cladding tube, and that - ' the total number of fuel assemblies in the whole core with said protective layer is smaller than 1.2 x C x 4 x N, where C is the average number of operating cycles during which a fuel assembly is allowed to sit in the core, and N is the number of control rods in the first group. 7. A core according to claim 6, characterized in that all the fuel assemblies with said protective layers contain a first group of fuel rods in which the inside of the cladding tube is provided with a protective layer, and a second group of fuel rods which have no such protective layers, and wherein said first group of fuel rods lies nearer the cen¬ tral part of the control rod that does said second group. 8. A core according to claim 6 or 7 , characterized in that the number of fuel rod positions in the main pat of the fuel assemblies is greater than 80. 9. A core in a pressurized-water reactor comprising a plura¬ lity of fuel assemblies (70a, 70b, 70c) , each one consisting of a bundle of fuel rods surrounded by a fuel channel, wherein each fuel rod comprises a cladding tube (20) which encloses a nuclear fuel (21), and certain fuel assemblies (70b, 70c) comprise, in addition, a plurality of fuel rods which may both be inserted into and withdrawn from the core, wherein a first group of fuel assemblies (70b) comprises control rods of which certain, during normal operation of the reactor, are at least partially inserted into the core, and wherein a second group (70c) of fuel assemblies compri¬ ses control rods which are all, during normal operation, withdrawn from the core, characterized in that - all the fuel assemblies (70b) in the first group comprise fuel rods in which the inside of the cladding tube is provi¬ ded with a protective layer of metal to prevent cladding damage caused by the interaction of the nuclear fuel with the cladding tube, and that - the total number of fuel assemblies in the whole core with said protective layer is less than 1.2 x C x N, where C is the average number of operating cycles during which a fuel assembly is allowed to sit in the core, and N is the number of fuel assemblies in the first group. 10. A core according to claim 9, characterized in that all the fuel assemblies with said protective layers contain a first group of fuel rods where the inside of the cladding tube is provided with a protective layer, and a second group of fuel rods which have no such protective layer, and wherein said first group of fuel rods lies nearer to a control rod than does said second group.

Inventors: NYLUND OLOV, NYLUND, OLOV
Applicants: ASEA ATOM AB, NYLUND OLOV, ABB ATOM AB, NYLUND, OLOV
Application year: 1994
Main classification: G21C3
IPC codes: G21C3